Soil represents a compartment of utter importance within the context of carbon (C) cycle since constitutes the major C reservoir at global scale, in particular in forest ecosystems, and regulates the
processes of the litter decomposition and immobilization of the organic matter controlling the mechanisms of carbon dioxide (CO2) exchange between the soil interface and the atmosphere. Soil is hence a fascinating compartment on one hand and complex on the other hand with regards to the
processes of biochemical nature, their measurements, the several feedbacks and their modellization.
The process of litter decomposition is the first step toward the C sequestration within the soil and it is mediated mainly by the activity of microorganisms; the interaction and stabilization within mineral matrix follows. Markedly the biotic component constitutes the most sensitive item to
external disturbances of both climatic rather than anthropogenic nature. Presently a lot of attention has moved toward the role of nitrogen (N) and the biochemical effects on the soil decomposition activity and C stock, but the experimental evidence is still not coupled to a general scheme able to
explain consistently all the results reported in literature. The present work focuses hence on the dynamics of C and N in to the soil, in particular the effects of a surplus of mineral N on C cycle
through the role of microbial community. In a first phase of the study a conceptual model have been proposed after an initial work of review; a more detailed investigation followed by medium of
different approaches, including an incubation experiment of a N-fertilized soil and the development of two models. The general introduction has underlined how soil effectively constitutes a crucial
issue concerning its contribute to C cycle within forest ecosystems, the response of which to external disturbance such as N deposition or N fertilization is markedly mediated by the significant
heterogeneity and hence by the spatial and the temporal scales of the processes. However, a conceptual scheme has been proposed in order to organize all the main findings in literature in a
general scheme able to include the observed microbial response to mineral N addition, and more generally to the availability of C and N of different quality. In this conceptual model microorganisms have been considered as a single individual that behaves in order to achieve an
optimal and efficient condition, depending on the energetic status of resources regardless the real mechanisms affecting any changes of C and N quality.
A part of the work interested the realization of an incubation experiment of a Mediterranean forest (Quercus cerris) soil subject to N addition. The type of site and the experimental set up with the
slow addition of mineral N constituted a novelty within the literature with regards to N fertilization studies in laboratory. In fact this study allowed firstly filling the gap concerning the current knowledge that interested mainly investigations on the forests of North Europe and North America.
Although the experiment was conducted in laboratory the soil showed in the first period a response similar to the response observed in a site covered by an oak-type vegetation in north America, supporting the hypothesis that on old organic matter originated by low-quality litter the
recalcitrance may control microbial response to N addition independently on direct control of climatic factors. Indeed these factors regulate mainly the vegetational composition, the litter quality
and the rate at which litter is initially degraded. Moreover it has been shown as the process depended on the microbial activity pattern throughout incubation time that affected in a non-linear
way the results at the end of incubation. The results suggested how mineral N may be used as keyvariable driving the microbial activity on the long temporal scale, overcoming the need to go into
deep of specific mechanisms of biochemical nature (i.e. acidification process).
A litter decomposition and organic matter accumulation model was thus developed, starting from a soil C basic scheme. The N cycle was modified in order to take into account the all interactions
between the two cycles, in particular with regards to N in both organic and mineral form. The goal of the work was to propose a simple formulation that expresses the N-control on long temporal scale and that can be included in the current dynamic soil model. The merit of the work was to the
modelling of a process presently not yet completely included in dynamic models. Indeed the effort was to find a trade off between the complexity of the biochemical processes and the modelling
approach than could maintain the most judicious mathematical simplification in order to be manageable and easy to be interfaced with the models currently present in literature. The proposed formulation modifies the decomposition activity as a function of microbial N demand and mineral
N availability in to the soil. The model, characterized by a paucity of parameters to be optimized was able to reproduce the experimental evidences reported in literature and underlined the role of
soil microorganisms and of the mineral N on the decomposition activity of recalcitrant matter.
However it was underlined how the model should need an optimization “ad hoc” in order to be extended to the whole soil profile. Regardless the optimization procedure the proposed model offers
an interesting formulation that can be included in the current plant-soil dynamic models.
Finally the last phase of the work was moved toward the modification of a soil model in order to consider the microbial community behaviour on mixed substrates, following the input match law and the achievement of a microbial goal, i.e. maximimation of growth. Population behaviour has
been investigated with regards to animal versus resources acquisition at high scales, but it has been rarely applied to the dynamic of soil C-cycle. The latter model, althoght still a theoretical exercise,
returned the best performances and adds thus a new dimension to the area of study of soil
decomposition models at the plot scale....more

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